The present invention relates to a method of microlithography using an optical exposure system to project mask images for use in the fabrication of semiconductor devices.
Optical lithography involves the creation of relief image patterns through the projection of radiation within or near the UV visible portion of the electromagnetic spectrum. Techniques of optical microlithography have been used for decades in the making of microcircuit patterns for semiconductor devices. Early techniques of contact or proximity photolithography were refined to allow circuit resolution on the order of 3 to 5 xcexcm. More modern projection techniques minimize some of the problems encountered with proximity lithography and have lead to the development of tools that currently allow resolution below 0.15 xcexcm.
Semiconductor device features are generally on the order of the wavelength of the ultraviolet (UV) radiation used to pattern them. Currently, exposure wavelengths are on the order of 150 to 450 nm and more specifically 157 nm, 193 nm, 248 nm, 365 nm, and 436 nm. The most challenging lithographic features are those which fall near or below sizes corresponding to 0.5 xcex/NA, where xcex is the exposing wavelength and NA is the objective lens numerical aperature of the exposure tool. As an example, for a 248 nm wavelength exposure system incorporating a 0.60NA objective lens, the imaging of features at or below 0.18 micrometers is considered state of the art. FIG. 1 shows the configuration of a projection exposure system. Such an exposure system can be used in a step-and-repeat mode (referred to a stepper tool) or in a step-and-scan mode (referred to as a scanner tool). A UV or vacuum ultraviolet (VUV) source 1 is used to pass radiation through the illumination system 2 using a condenser lens system 3 and a fly""s eye microlens array 4. An aperture 5 shapes the illumination profile to a defined area and radiation is relected from a mirror 6 to pass through an illumination lens 7 to illuminate a photolithographic mask 8. Upon illumination of the photomask 8, a diffraction field 11 distributed as spatial frequency detail of the photomask 8 is directed through the objective lens 9 to be imaged onto the photoresist coated semiconductor substrate 10. Such an exposure system forms an image by collecting at least more than the 0th-order of the diffraction field from the photomask 8 with the objective lens 9. The absolute limitation to the smallest feature that can be imaged in any optical system corresponds to 0.25 xcex/NA. Furthermore, the depth of focus (DOF) for such an exposure tool can be defined as +/xe2x88x92k2xcex/NA2 where k2 is a process factor that generally takes on a value near 0.5.
As geometry sizes continue to shrink below 0.5 xcex/NA, methods of resolution enhancement are being required to ensure imaging with adequate fidelity and depth of focus. Such methods of resolution enhancement developed over recent years can allow for improvement in addition to those made possible with shorter exposing wavelengths and larger numerical apertures. Off-axis illumination (OAI) and phase-shift masking (PSM) are current examples of resolution enhancement techniques.
By using OAI in a projection imaging system, image refinement is carried out by considering illumination apertures which are not necessarily circular. In a system where illumination is obliquely incident on the mask at angles so that the zeroth and first diffraction orders are distributed on alternative sides of the optical axis, two diffraction orders are sufficient for imaging. An illumination angle can be chosen using two uniquely placed circular poles (dipoles) for a given wavelength, NA, and feature size. This is shown for example in the prior art of FIG. 2, where the normalized angular distribution of illumination (sin xcex8/NA)is represented. This illumination angle resulting from the two apertures, 20, is can be chosen for dense features as sin xcex8=(0.5 xcex/p) where p is the feature pitch. The most significant impact of this dipole off axis illumination is realized when considering focal depth. In this case, the zeroth and first diffraction orders travel a more similar path length compared to conventional illumination as defocus is considered.
Off axis illumination using dipole illumination, oriented in the direction of mask geometry, can offer the most significant enhancement to imaging performance. This is because only oblique illumination at an optimized illumination angle can be designed to allow projection of mask diffraction energy at the outermost edges of an objective lens pupil. Frequency doubling is made possible (at the limit where pole size approaches zero and point source behavior occurs) and extreme focal depth can be achieved (since radial usage of the objective lens pupil in limited to a narrow region near the outside edge). The problem with dipole illumination arises when geometry of both X and Y (or horizontal and vertical) nature is considered. In practice, by limiting illumination to allow for one narrow beam or pair of beams leads to zero intensity. Also, imaging is limited features oriented along one direction in an X-Y plane. To overcome this, an annular or ring distribution has been historically employed which delivers illumination at angles needed with a finite ring width to allow for some finite intensity [see for instance H. H. Hopkins, Proc. Royal Soc. A, Vol. 217, 408-432 (1953)]. The resulting focal depth is less than that for the ideal case but improvement over a full circular aperture can be achieved.
For most integrated circuit applicatons, features are limited to X and Y orientations only and a four pole or quadrupole configuration can be more suitable (see for instance U.S. Pat. No. 5,305,054). In these cases, a quadrupole type illumination is required to accommodate the two orthogonal orientations of mask features. Solutions for such quadrupole illumination is where poles are at diagonal positions oriented 45 degrees to X and Y mask features. This is shown in the prior art of FIG. 3. Here, each illumination pole, 30, is off axis to all mask features and image improvement for X and Y oriented features occurs. The maximum angle for this quadrupole illumination is limited compared to a dipole illumination because of the placement of poles on diagonal axes. The maximum illumination angle is smaller than that for the dipole configuration by a factor of the square root of two. Resolution or imaging potential is also reduced as compared to dipole type off-axis illumination by this factor of the square root of two. This diagonally oriented quadrupole approach to off-axis illumination, and variation on this approach including weak Gaussian-pole designs, has been used for optical microlithography applications for several years now. Imaging below 0.4 xcex/NA (for 1:1 line to space ratio geometry) has not been demonstrated using this approach however.
Phase shift masking has been use for several years to improve lithographic imaging [see for instance Levenson et ale, xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d, IEEE Transactions on Electron Devices, vol. ED-29, No. 12, p. 1828-, December 1982]. With conventional binary masking, only the control of the amplitude of a mask function is considered and phase information is assumed to be non-varying additional manipulation of phase information at the mask can allow for improvement of imaging performance. For coherent illumination, when a xcfx80 xe2x80x9cphase shifterxe2x80x9d is added at alternating mask openings in a mask, an objective lens pupil has a 50% decrease in required numerical aperture required to capture required diffraction orders. Alternatively, for a given lens numerical aperture, a mask which utilizes such alternating aperture phase shifters could image features one-half the size that is possible using a conventional binary mask. As partial coherence is considered, the impact of this phase shift technique is diminished to a point where for incoherent illumination no improvement is realized for phase shifting over the binary mask. This technique of phase shifting alternating features on a mask is appropriately called alternating phase shift masking. Phase information is modified by either adding or subtracting xe2x80x9copticalxe2x80x9d material from the mask substrate at a thickness which corresponds to a xcfx80 phase shift.
Phase shift masking has been used to improve resolution and focal depth in projection lithography. Several types of phase shift masks are known including a chromeless or phase shift mask, which is shown in the prior art of FIG. 4 [see K. Toh et al. xe2x80x9cChromeless Phase-Shifted Masks: A New Approach to Phase-Shifting Masksxe2x80x9d, SPIE, vol. 1496, p. 27, 1990]. In this case, the boundaries of a large phase pattern are utilized as single dark imaging features through the production of localized destructive intensity regions at the imaging plane. Globally dark regions have be produced by placing many single phase edges in close proximity on the mask through use of checkerboard and other repetitive structures. Fine feature resolution (below 0.4 xcex/NA) becomes difficult to demonstrate using this single phase boundary approach, as does the imaging of non-isolated geometry. Significant sizing biases have been also been required.
In view of the above described problems with the prior techniques of image resolution enhancement, there is a need for a method to allow for lithographic resolution at or below 0.4 xcex/NA. The art also needs a method for lithographic imaging that combines an off-axis illumination and phase shift masking in such a way as to avoid the adverse problems associated with each to achieve resolution at or below 0.4 xcex/NA, and a method that produces results that cannot be obtained without the said combination of the illumination and masking. The present invention provides a phase-shift mask which has two phase sifting boundaries or edges in close proximity so that the two edges form a single small dark region during image formation. The invention also provides an off-axis condition of illumination which places four poles on axis to accommodate the phase shift mask consisting of two phase shifting boundaries or edges in close proximity so that the two edges form a single small dark region during imaging, where a satisfactorily high degree of image contrast exists for image formation.
The present invention provides an imaging method for producing fine lithographic features oriented along two orthogonal directions. It includes an illumination source having four separate localized areas, each area having higher transmittance than portions of said illumination surrounding said areas. The areas are arranged at locations on the two orientation axes of the lithographic patterns and at a separation distance corresponding to the frequency of the fine pattern detail. The mask of the present invention is a phase shifting mask which comprises a transparent substrate and a phase shift formed into said substrate by etching the substrate to form fine phase shift features. The phase shift feature boundaries are spaced close together so that the destructive image intensities at the imaging plane for each feature are not individually resolved but instead produce single dark region by the overlap of destructive image intensity from the two boundaries during image formation using the said illumination source. The mask can include a transparent phase shift layer that comprises a material that is transparent and is a thickness so that a phase shift occurs. The phase shift layer is patterned by etching the layer to form the phase shift features and is used in combination with the said illumination source.
In the above described invention, a quadrupole illumination approach is used. It produces four poles placed on axial positions for off-axis illumination of X and Y oriented mask geometry, which we refer as xe2x80x9ccross-quadxe2x80x9d quadrupole illumination. This illumination method is not adequate for imaging improvement using conventional masking techniques because a two-pole pair on a given axis produces optimal off-axis illumination for the corresponding feature orientation while producing undesirable illumination for the orthogonal orientation. The problem can be understood if mask illumination and the resulting diffraction effects are considered, as shown in FIGS. 5A through 5C. An example of cross-quad illumination is shown in FIG. 5A, where poles, 40, are placed on illumination axes. As shown in the diffraction field of FIG. 5B for one mask feature orientation, the two optimized off-axis illumination pole pairs produce the intended off-axis illumination and desired distribution of diffraction energy, 50, in the objective lens pupil, 52. However, poles in the opposing orientation introduce undesired illumination and distribution of diffraction energy, 53. This undesired illumination of the conventional mask features behaves as low partial coherence, a condition that is not well suited for fine feature geometry. The problem is worsened further because of there is a two non-optimal pole contribution to the illumination. When considering the projection of diffraction energy, 53, from these non-optimal poles into the objective lens pupil, 52, the diffraction amplitude is twice that of the diffraction energy at the pupil edge resulting from the optimal off-axis illumination, 50. This type of quadrupole off-axis illumination would therefore be avoided in favor of the a diagonal quadrupole or other approaches for conventional masking.
However, there is an inherent advantage to the cross-quad quadrupole approach, namely, the absence of the square root of two factor that exists with the diagonal quadrupole approach. That factor limits imaging potential of the diagonal quadrupole as compared to the dipole-type illumination. The cross-quad quadrupole illumination distributes off-axis illumination angles in the same manner as the dipole and equivalent diffraction energy distribution results. To realize this increased potential we have discovered that the cross-quad approach may be combined with the unique condition of phase boundary phase masking where two fine phase shift feature boundaries are produced in close proximity. We refer to this phase masking method as xe2x80x9cdual-boundary shiftingxe2x80x9d. The cross-quadrupole and phase shift mask captures the diffraction energy for useful illumination.
More specifically, we have invented a projection imaging method where small sub-wavelength features are imaged using cross-quad illumination designed so that poles along a single illumination axis place diffraction orders that overlap and are distributed on opposite sides of the optical axis at identical radial positions. The poles are centered with respect to the fall illumination pupil at a location which corresponds to sin xcex8=xcex/(2p), where p is the pitch or spacing between the closest features in the mask. This illumination is accomplished using an aperture filtering device (see U.S. patent application Ser. No. 09/422,398) a metal masking approach to aperture plane illumination filtering (see U.S. Pat. No. 5,305,054), a multiple beam splitter approach (see U.S. Pat. No. 5,627,625) or a diffractive optical element approach (see U.S. Pat. No. 5,926,257 or see U.S. Pat. No. 5,631,721). This illumination method is combined with dual-boundary shifting phase-shift masking where chromeless mask features are designed by placing phase edges at a separation distance smaller than the diffraction limit of the projection imaging system.